"I think what's important for kids to know is that your decisions here on earth matter, your behavior matters, and how you treat other people matters," Damon said. "It matters here and I'm hoping that it matters in the afterlife, too. It just comes down to accountability for your own behavior that's important. So whenever I talk to my own kids or whenever they ask, I try and put the hereafter in kind of an ethical framework."

Damon also reinforces the "line in the sand" that he and his wife Luciana have drawn when it comes to having more kids.

"This is it," he says. "Our lives are full and wonderful and we're done having kids."

Though the actor is done expanding his family, a friend of Damon's tells PEOPLE he's over the moon about the latest addition to the clan.

"He just had a 40th birthday party in N.Y.C., and he and [Luciana] just looked so very much in love and [they are] just delighted about the baby ... I am really happy for him.”